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    <title>Document</title>
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<body>
    <script>
        let s = new Array(6)
        console.log(s);
        let s2 = new Array(6,7)
        console.log(s2);

        var s3;  //undefined数据类型==唯一的值undefined typeof
        var s4 = new Student()
        s4 = null  //s4在这种情况,数据类型null类型,值为null
        
        // undefined是null的派生类(子类)
        console.log(undefined == null);  //true
        console.log(undefined === null);  //false
    </script>
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